1. Field of the Invention
The present invention relates to an image exposure apparatus used with a copying machine, a printer and the like, and an image forming apparatus having such an image exposure apparatus, and more particularly, it relates to an image exposure apparatus in which an image is exposed by lightening a plurality of luminous elements such as LEDs.
2. Related Background Art
In conventional image forming apparatuses having an array of light emitting diodes (referred to as xe2x80x9cLEDxe2x80x9d hereinafter) as an exposure source, an photosensitive drum is exposed by light emitted from the LED array and an image is formed on the photosensitive drum by an electrophotographic process. FIG. 12 schematically shows such as LED array. As shown in FIG. 12, a plurality of LED chips 101 are disposed on a substrate 102 of an LED array 100 equidistantly along a direction parallel with a rotational axis of a photosensitive drum (not shown). A length of the LED array 100 is determined by a length of the photosensitive drum. As shown in FIG. 13, each LED chip 101 includes a plurality (normally, 64 to 128) of light emitting points. FIGS. 14A and 14B show sections of the LED chip 101. The LED arrays are generally divided into two groups, i.e. GaAs group and AlGaAs group which have different features.
FIG. 14A shows the LED array of GaAs type wherein GaAsxP(1-x) of n-type are formed on a GaAs substrate of n-type by a gas phase crystal growth method. In this case, as the rate of P is increased, a light emitting wavelength is lengthened to increase light emitting efficiency. A luminous junction is formed by forming a p-area in an n-GaAsP layer by thermal diffusion of zinc (Zn). An interface of the p-n junction acts as a light emitting diode. In general, in order to define the diffusion of zinc within a limited area of the light emitting portion, a film of SiO2 is formed in an opening portion, and density of carrier is controlled through the film to effect the diffusion of zinc. P-electrodes for applying current are made of aluminium (Al) or Auxe2x80x94Sexe2x80x94Te alloy (gold/selenium/tellurium alloy) and an n-electrode is common to the arrays and is made of Au/Auxe2x80x94Gexe2x80x94Ni (gold/gold-germanium-nickel).
The LED is an element for applying voltage to the p-n junction in a normal (positive) direction and for pouring small amount of carrier and for picking up natural light generated by re-binding of carrier. In order to improve the light emitting efficiency, it is important that internal quantum yield for converting the applied current into the light is maximized by utilizing direct transition to the re-binding process and that the emitted light is efficiently taken out to the exterior. The efficiency for taking out the light to the exterior (external quantum yield) is several percentage (%) or less since there are components entirely reflected into the interior of the semiconductor at a critical angle determined by refractive index of material or substance, and, thus, a major part of light is absorbed into the interior and consumed as heat. Accordingly, in the LED array, it is important that the efficiency of the internal quantum yield is improved by purifying the crystal and at the same time the efficiency of the external quantum yield is increased.
FIG. 14B shows the LED array of AlGaAs type wherein AlGaAs is formed on a GaAs substrate of p-type by a liquid phase crystal growth method. In this LED array, a mixture ratio between gallium (Ga) and aluminium (Al) can be controlled within a wide range. First of all, a p-layer of Al(1-x1)Gax1As on the p-substrate is grown, and then, an n-layer of Al(1-x2)Gax2As is grown, thereby forming a p-n junction portion between the layers. By changing x at the interface of the junction, it is possible to form heterojunction and to make the current applying efficiency (i.e., the re-binding contributing to the light emission) more effective. Further, since the value of x2 can be selected as a transparent layer having less light-absorbing feature with respect to a light taking-out direction, it is possible to take out a larger amount of external light emitting output. Incidentally, the common electrode of p-side is made of AuZnxe2x80x94Nixe2x80x94Au (gold/zinc-nickel-gold) and the electrode of n-side is made of AuGexe2x80x94Nixe2x80x94Au (gold/germanium-nickel-gold) and these electrodes become ohmic electrodes.
The LED array is formed by arranging the plural LED chips as mentioned above side by side on the substrate 102 (die bonding). All of the light emitting elements (pixels) in the LED chip 101 are connected to corresponding wires (wire bonding). The LED (light emitting element) is illuminated by applying current to the corresponding wire. The light emitting points 103 are equidistantly disposed in the chip. Since the pixels are associated with the wires one by one, for example, when there are 128 light emitting points 103 in one LED chip 101, the number of the wire bondings becomes 128. FIG. 15 is a perspective view showing a condition that the LED chips 101 are mounted on the substrate in this way and the light emitting elements in the LED chips are connected to drivers by wire bondings.
Next, a method for driving the LED will be explained.
In order to drive the LED, generally, a driving method utilizing constant current driving elements is used. The constant current driving methods are generally grouped into two methods as shown in FIGS. 18A and 18B. In the first method, internal resistance is added to a P-channel open drain CMOS circuit or serial resistance as external resistance is added (serial resistance type). In the second method, a constant current circuit is provided by controlling a gate voltage of a driver IC. The second method having less current fluctuation in comparison with the first method is more preferable for voltage fluctuation. In FIG. 18A, the current is made constant by base current Q1 of transistor Q2, thereby controlling the driving current of the LED. On the other hand, in FIG. 18B, false constant current is established by high resistance R.
Methods for inputting a signal are generally grouped into four, as shown in FIGS. 19A to 19D. In the methods shown in FIGS. 19A and 19B, signals are successively supplied to shift registor(s) and are latched upon illumination, and an output signal is time-controlled by an enable signal, thereby determining a time period for illuminating the LED. The difference between FIGS. 19A and 19B is that the entire head is constituted by a single serial shift register (FIG. 19A), whereas, the signals are supplied, in parallel, to a plurality of input terminals of plural shift registers.
In the method shown in FIG. 19D, the division is effected every one dot, and this method apparently bears resemblance to a parallel input method shown in FIG. 19C.
FIG. 19C shows the complete parallel input fashion, in which the data are always inputted to the head in parallel and the light emitting position is determined by the timing of the latches. Now, the method shown in FIG. 19C will be further fully explained. Eight-bit parallel signals are inputted to n-th (n=0 to 7; eight in total) ports in accordance with the latch signal of the data, and the 8-th to 15-th data are read by the next clock input. After all of the data are latched, the data are transferred to another latch portion, where light emitting time period for illuminating the LED is determined.
Regarding the characteristics shown in FIGS. 19A, 19B and 19C, in FIG. 19A, the maximum speed is limited by a transmitting speed of the shift register, and in FIGS. 19B and 19C, since the time period is reduced to 1/n (n is the number of the input ports), the high speed operation can be expected. Particularly, the circuit shown in FIG. 19C is suitable for the highest speed operation since there is no data transmission.
In any cases, in the final output stage, in the case where the light emitting dots in the LED is great, particularly, when all of the light emitting elements are illuminated simultaneously, even if a single dot is illuminated by current of 5 mA, since 3000 to 4000 dots are illuminated simultaneously, large current in the order of 15 to 20 A will be applied to the entire head. Accordingly, the resistance values of a power source and an earthing wire which constitute a common line must be decreased. Although the LED can be sufficiently driven by the current of 5 A, electric power of 100 W (=5 mAxc3x975 Vxc3x974000) is consumed in the head. Thus, adequate cooling is required for heat.
However, in the arrangement wherein the pixels correspond to the wires one by one, as the density of the pixels is increased, since the dimension of each pixel is decreased, the density of the wire bondings is also increased. As a result, there arises a problem that the adjacent wires are contacted with each other and/or the wire is broken since due to fineness. Since it is considered that the density of the LED pixels will be further increased, the above problem must be solved.
To solve the above problem, there has been proposed a technique in which the shift register is mounted on the LED element itself so that the light emitting points 103 in the LED is successively transferred from LED chip to LED chip. By using the LED of this kind, since there is no need to connect the wires to the LED pixels one by one, even when the density of the LED pixels is increased, the number of the wire bondings can be greatly reduced.
However, when a straight line having a length greater than the single LED chip 101 is written along a main scan direction by using the shift register mounted LED array as an exposure means of an electrophotographic image forming apparatus, since the transferring speed of the light emitting point 103 of the LED and a rotational speed of a photosensitive drum of the image forming apparatus are limited, it is feared that an exposure line on the photosensitive drum is deviated from the main scan direction to distort the straight line.
Further, although such distortion can be eliminated by increasing the transferring speed of the light emitting point 103 of the LED, if do so, since the light emitting time period of each pixel is decreased to reduce the exposure amount. This is not preferable. Further, since the transferring speed of the light emitting point 103 is limited, the LED chips 101 themselves are subjected to load.
The present invention aims to eliminate the above-mentioned conventional drawbacks, and an object of the present invention is to provide an image exposure apparatus and an image forming apparatus having such an exposure apparatus, which can prevent deviation or shift of exposure point on a photosensitive member.
Another object of the present invention is to provide an image exposure apparatus and an image forming apparatus having such an exposure apparatus, in which the number of wire bondings is reduced and degree of image dissector is improved.
A further object of the present invention is to an image exposure apparatus in which a plurality of light emitting elements are disposed on a substrate and these light emitting elements are arranged along a longitudinal direction of the substrate in a nonparallel relation.
A still further object of the present invention is to an image forming apparatus comprising a photosensitive member, and an exposure means including a plurality of light emitting elements to expose the photosensitive member. Wherein the plurality of light emitting elements are disposed in a non-parallel relation with respect to generatrix of the photosensitive member.
The other objects and features of the present invention will be apparent from the following detailed explanation of the invention.